Problem: In history class, the girl to boy ratio is $4$ to $6$. If there are a total of $40$ students, how many boys are there?
Solution: A ratio of $4$ girls to $6$ boys means that a set of $10$ students will have $4$ girls and $6$ boys. A class of $40$ students has $4$ sets of $10$ students. Because we know that there are $6$ boys in each set of $10$ students, the class must have $4$ groups of $6$ boys each. There is a total of $24$ boys in history class.